1. Technical Field
The present invention relates to a burner for manufacturing a porous glass material having good deposition efficiency.
The contents of the following Japanese patent applications are incorporated herein by reference,
No. 2008-046833 filed on Feb. 27, 2008,
No. 2009-034224 filed on Feb. 17, 2009,
No. 2009-042296 filed on Feb. 25, 2009, and
PCT/JP2009/000884 filed on Feb. 27, 2009
2. Related Art
Various conventional methods have been proposed for manufacturing an optical fiber base material. One such method is OVD (Outside Vapor Phase Deposition), which involves relatively moving the burner or a starting member back and forth to affix and deposit glass fine particles generated in the burner flame onto the rotating starting member to synthesize the porous base material, and dehydrating and sintering this base material in an electric furnace. With this method, an optical fiber base material having a relatively arbitrary refractive index distribution can be obtained and mass production of optical fiber base materials with large aperture diameters can be achieved, and so this method is commonly used.
FIG. 1 is a schematic view showing an exemplary porous glass base material manufacturing apparatus that uses the OVD method. In FIG. 1, the starting member on which the glass fine particles (soot) are deposited is realized by dummy rods 2 fused at both ends of a core rod 1, and the ends of the dummy rods 2 are supported by ingot chuck mechanisms 4 to be rotatable on an axis. The optical fiber raw material, vapor such as SiCl4, and a combustion gas such as hydrogen gas and oxygen gas are blown toward the starting member from the burner 3 that moves back and forth relative to the starting member, and the optical fiber porous base material is formed by depositing on the starting member the soot generated by the hydrolysis in the oxygen flame. Here, reference numeral 5 indicates an exhaust hood.
The burner 3 is supported to move back and forth in the longitudinal direction of the starting member via a burner guide mechanism, not shown. While the starting member rotates on an axis, the burner blows the flame toward the starting member, thereby forming the porous base material by depositing glass fine particles generated by the hydrolysis of the raw material gas in the flame. Next, the porous base material is passed through a heater of a heating furnace, not shown, to become dehydrated glass, thereby forming the optical fiber base material.
In order to synthesize the glass fine particles and deposit the soot on the starting member, a burner having a plurality of coaxial pipes is conventionally used. However, such a burner is unable to generate a sufficient amount of glass fine particles, since there is insufficient mixing of the glass raw material gas, the combustion gas, and the auxiliary combustion gas. As a result, the yield cannot be increased and the high-speed synthesis becomes difficult.
In order to solve this problem, Japanese Examined Patent Application Publication No. 03-9047 proposes a multi-nozzle burner having a plurality of nozzles within the combustible gas discharge port that form small aperture diameter auxiliary combustion gas discharge ports arranged to surround the central raw material gas discharge port. With this type of burner, several methods for improving deposition efficiency are proposed. For example, Japanese Patent Application Publication No. 2003-206154, Japanese Patent Application Publication No. 2004-331440, Japanese Patent Application Publication No. 2006-182624, and Japanese Patent No. 3744350 propose small aperture diameter auxiliary combustion gas discharge ports. Furthermore, Japanese Patent Application Publication No. 05-323130, Japanese Patent No. 3543537, and Japanese Patent Application Publication No. 2003-226544 describe optimization of the focal distances of the small aperture diameter auxiliary combustion gas discharge ports. Japanese Patent No. 3591330, Japanese Patent Application Publication No. 2003-165737, Japanese Patent Application Publication No. 2003-212555, and Japanese Patent No. 3653902 describe optimizing the gas flow rate and gas linear velocity.
The inventors of the present invention performed a rigorous investigation of a burner for manufacturing a porous glass base material having small-diameter auxiliary combustion gas discharge ports, i.e. nozzles. As a result, it was found that the deposition efficiency is strongly linked to the configuration and focal distances of the small-diameter auxiliary combustion gas discharge ports, the gas flow rate, and the gas linear velocity. However, there have been problems such as a non-uniform reaction caused by a variation of the gas linear velocity in the gas discharge ports and disruption of the flame caused by unstable gas flow, and these problems interfere with improvements to the deposition efficiency.
Usually, the supply of a reaction gas to the burner involves disposing gas inlet pipes at certain locations near the source side of the pipes forming the gas discharge ports, and supplying the reaction gas to each gas discharge port via supply tubes connected to the gas inlet pipes. The gas supplied to the gas discharge ports is supplied from gas inlet pipes connected at certain locations in a direction orthogonal to a ring-shaped (annular) flow path, but is not supplied to the central gas discharge port. Here, since the burner is configured as multiple coaxial pipes, pipes farther outward have larger diameters, and the gas supplied from the gas inlet pipes to the ring-shaped flow path has difficulty flowing around on an opposite side of the inner pipes when these pipes are positioned farther outward. As a result, it is easy for the linear velocity in a flow path cross section of the gas discharge ports to become non-uniform.
In particular, a burner having small-diameter auxiliary combustion gas discharge ports has a group of the small-diameter auxiliary combustion gas discharge ports arranged within one gas discharge port, and so compared to a conventional burner with coaxial pipes that does not have small-diameter auxiliary combustion gas discharge ports, a variation of the linear velocity of the gas from the gas discharge ports is more likely to occur. Therefore, a method is considered for reducing linear velocity variation within the gas discharge ports by disposing a plurality of gas inlet pipes in the pipes forming the gas discharge ports, but this is difficult to realize because a large number of gas inlet pipes are necessary, resulting in a very complex configuration.
It is an object of the present invention to provide a burner for manufacturing porous glass base material that has small-diameter gas discharge ports and that can achieve uniform linear velocity, a uniform reaction, a stable flame, and improved deposition efficiency.